Conventionally, a combustion apparatus, such as an internal-combustion engine, uses a spark plug for igniting an air-fuel mixture through spark discharge. In recent years, in order to meet demand for high output and low fuel consumption, a plasma jet ignition plug has been proposed, because the plasma jet ignition plug provides quick propagation of combustion and can more reliably igniting even a lean air-fuel mixture having a higher ignition-limit-air fuel ratio.
Generally, the plasma jet ignition plug includes a cylindrical insulator having therein an axial bore, a center electrode inserted into the axial bore in such a manner that a front end surface thereof is located internally of a front end surface of the insulator, a metallic shell disposed externally of the outer circumference of the insulator, and an annular ground electrode joined to a front end portion of the metallic shell. Also, the plasma jet ignition plug has a space (cavity) defined by the front end surface of the center electrode and an inner circumferential surface of the axial bore. The cavity communicates with an ambient atmosphere via a through hole formed in the ground electrode.
Additionally, such a plasma jet ignition plug ignites an air-fuel mixture as follows. First, voltage is applied to a cavity formed between the center electrode and the ground electrode, thereby generating spark discharge therebetween and thus causing dielectric breakdown therebetween. In this condition, electrical energy is applied to the cavity so that a gas in the cavity becomes a plasma state and plasma is generated within the cavity. The generated plasma is discharged or jetted through an opening of the cavity, thereby igniting the air-fuel mixture.
Meanwhile, a known ignition apparatus for a plasma jet ignition plug includes: a voltage application portion for applying voltage to the cavity and causing spark discharge; and a power supply portion for supplying electric power energy to the cavity (e.g., refer to Japanese Patent Application Laid-Open (kokai) No. 2010-218768 “Patent Document 1”).